Wireless communication systems are widely known in which a base station communicates with multiple subscriber stations or users within range of the base station. The area covered by one base station is called a cell and, typically, many base stations are provided in appropriate locations so as to cover a wide geographical area more or less seamlessly with adjacent cells. In conventional such cellular wireless networks the equipment of each user (“user equipment” or “UE”) is only served by one base station (BS) at a time. However, this can result in low cell-edge data rates and coverage owing to high inter-cell interference at the cell-edge. To reduce the cell-edge interference it is beneficial to serve a cell UE by multiple base stations; this is termed “multi-cell multiple-input/multiple-output” or “multi-cell MIMO”. By using multi-cell MIMO the harmful interference from neighbouring cells can be turned into useful signals, thereby improving cell-edge throughput, system throughput and coverage.
However, coordinating the multiple-input multiple-output (MIMO) transmissions among multiple base stations requires channel knowledge and data information to be shared among the coordinated base stations, resulting in additional requirements on the backhaul capabilities. Furthermore, for FDD systems, the channel knowledge is mainly obtained by UE feedback. Since multiple cells participate in the coordinated transmission, the amount of channel knowledge needed at the network side increases linearly with the number of cooperating cells, which will be a heavy burden for the uplink channel. Therefore, in practice, in order to relax the backhaul burden and improve the efficiency of cooperative multi-cell transmission, it is usually better to use only some of the cells which could potentially be used to cooperatively serve UEs on a given time-frequency resource. In such a case, it is necessary to determine which of the cells have priority, over the other cells, to be used cooperatively to serve a given UE.
As an example, FIG. 1 of the accompanying drawings illustrates an application scenario of a cell grouping method based on the terminology and definitions given by LTE-A (see R1-092290, “TP for feedback in support of DL CoMP for LTE-A TR”, Qualcomm Europe, 3GPP TSG-RAN WG1 #57, 4-8 May, 2009, San Francisco, USA). It should be noted that the LTE-A system serves purely as an example and the invention could be applied to any other multi-cell MIMO system.
The 3GPP standardisation body has identified coordinated multi-point transmission/reception (CoMP) as a key technology that is included in the LTE-A study item to improve the coverage of high data rates, the cell-edge throughput and/or to increase system throughput. Essentially CoMP is a coordinated multi-cell MIMO transmission/reception scheme, and, according to 3GPP TR 36.814 “Further Advancements for E-UTRA Physical Layer Aspects (Release 9)”, V1.0.0, 2009-02-26, its downlink scheme is mainly characterized into two categories, termed                “Coordinated scheduling and/or beamforming (CS/CB)” and        “Joint Processing/Transmission (JP/JT)”.        
In the category of CS/CB, “data to a single UE is instantaneously transmitted from one transmission point, but user scheduling/beamforming decisions are made with coordination among cells corresponding to the CoMP cooperating set”, while in the category of JP, “data to a single UE is simultaneously transmitted from multiple transmission points to (coherently or non-coherently) improve the received signal quality and/or cancel interference for other UEs”.
In the example of FIG. 1, it is assumed that cells A, B and C actively transmit to a UE (termed CoMP Transmission Points), while cell D is not transmitting during the transmission interval used by cells A, B and C. The set of cells A, B, C and D is termed a “CoMP Cooperating Set”. They can be selected by using some selection principles based on the measurements provided by the cells in the measurement set. For example, recently some cell selection methods were proposed to configure the CoMP cooperating set based on the RSRP measurements of the cells in the measurement set (see R1-092833, “Discussions on CoMP cooperating set”, CHTTL, 3GPP TSG-RAN WG1 #57bis, 29 Jun.-3 Jul., 2009, Los Angeles, USA and PCT/EP2009/006572 filed on 10 Sep., 2009). In order to obtain certain target transmission gains by using CoMP (e.g. target signal to interference plus noise ratio (SINR), target transmission data rate), some of the cells of the CoMP cooperating set are grouped together to carry out certain cooperative transmission. For example, if cell B is assumed to be the serving cell of a given UE since it can provide the highest average received signal power, and additionally if either cell A or cell C can be grouped with cell B to meet the target transmission data rate, in such a case it is beneficial to have a criterion to decide which cell has the priority to be grouped with cell B, because properly grouping cells that potentially carry out cooperative transmissions can help reduce the feedback overhead needed during the coordinated transmissions, which is a significant problem encountered by CoMP for the uplink channel.
It should be noted that CoMP may not be suitable for high mobility scenarios, such as where the velocity of the UE is ≧120 km/h, since high performance CoMP solutions are very sensitive to channel information being outdated (see R1-084322, “Scalable CoMP solutions for LTE advanced”, Nokia Siemens Networks, Nokia, 3GPP TSG-RAN WG1 #55, 10-14 November, Prague, Czech Republic) and the requirement on feedback is too difficult to be satisfied (see also R1-092309, “High-level principles for CSI feedback for DL MIMO and CoMP in LTE-A”, Alcatel-Lucent, Philips, Qualcomm, 3GPP TSG-RAN WG1 #57bis, 29 Jun.-3 Jul., 2009, Los Angeles, USA). Therefore, in the following, it is assumed that CoMP is intended for low mobility and medium mobility scenarios.
In the paper by A. Papadogiannis, D. Gesbert and E. Hardouin, “A dynamic clustering approach in wireless networks with multi-cell cooperative processing”, Proceedings of the IEEE international conference on communications, 2008 (ICC 2008), a dynamic cell clustering approach is proposed to form clusters of cooperating BSs for multi-cell cooperative processing (MCP). The clustering approach in this article is based on the sum-capacity maximization criterion, and the clusters can be dynamically formed in order to maximize the joint capacity of all the UEs within each cluster.
In the paper by X. Gao, A. Li, H. Kayama, “Low complexity downlink coordination scheme for multi-user CoMP in LTE-Advanced system”, Proceedings of the IEEE international symposium on personal, indoor and mobile radio communications, 2009 (PIMRC 2009), a method that can divide the cooperating set into best groups of transmission points and coordinated UEs is proposed for downlink CoMP MU-MIMO. Since it uses the large-scale channel information rather than the short-term channel information for CoMP MU-MIMO precoding and for grouping transmission points and UEs, the feedback overhead can be decreased during the operation of CoMP.
In the paper by M. Kamoun and L. Mazet, “Base-station selection in cooperative single frequency cellular network”, Proceedings of the IEEE workshop on signal processing advances in wireless communications, 2007 (SPAWC 2007), a BS selection algorithm is disclosed to be used in the context of uplink cooperative single frequency cellular network. In order to relax the fixed network load, a small number of BSs that can best decode the data for a given UE are selected based on the criterion of maximizing the uplink capacity of the given UE.
In a method proposed in the paper by S. Venkatesan, “Coordinating base stations for greater uplink spectral efficiency in a cellular network”, in proceedings of the IEEE international symposium on personal, indoor and mobile radio communications, 2007 (PIMRC 2007), a static clustering of BSs is used to prove the significant improvement in uplink spectral efficiency by coordinating BSs in reception of data from UEs. The cooperation clusters are static, and each UE is assigned to a certain cluster by using the highest SINR principle.
CN101389115A presents a method that can divide all base stations into base station clusters including fewer base stations, and executing collaborative communication between base stations in the same cluster. The selection of base stations by each UE and the allocation of UEs into each base station cluster are both according to strength of the pilot signal to ensure accurate communication between the collaborative base stations and the customers.
WO2009061660A discusses a method for selecting relay stations to perform either cooperative or non-cooperative communication with a UE. The selection of relay stations is based on threshold values using outage or throughput constraints, applied to both the base station to the relay station and the relay station to UE links.
Furthermore, in US2008247478A a method to select a reference relay station and a cooperative relay station among a plurality of relay stations is disclosed. The selection of the relay stations is based on location information of the UE and average CQI information. The information is sent from the relay stations to the base stations using a search request message.
It is therefore desirable to provide a method and apparatus which can help determine the active cells to join the coordination based on the criterion of reducing the feedback overhead needed during the operation of coordinated multi-cell transmissions.